Process of coating polymeric shaped objects with aluminum orthosphosphate

ABSTRACT

Organic polymeric shaped objects, such as films, are coated with aluminum orthosphosphate of a specific atom ratio, by a process for their preparation using an aqueous dispersion of phosphoric acid and aluminum chlorhydroxide.

United States Patent [1 Haskell et al.

' PROCESS OF COATING POLYMERIC SHAPED OBJECTS WITH ALUMINUM ORTI'IOSPIIOSPHATE Inventors: Vernon C. Haskell; James L. Hecht,

both of Richmond, Va.

Assignee: E. I. duPont de Nemours and Company, Wilmington, Del.

Filed: Jan. 3, 1973 Appl. No.: 320,752

Related US. Application Data Continuation-impart of Ser. No. 273,301, July 19, 1972, Pat. No. 3,821,014, which is a continuation-impart of Ser. No. 182,641, Sept. 22, 1971, abandoned.

US. Cl 117/46 FC, 117/47 A, 117/72, 117/76 F, 117/l38.8 F, 1l7/138.8 R,

Int. Cl C0lb 25/36, B44d 1/092 Field of Search 117/47 A, 169 R, 138.8 R,

[ 5] Dec. 31, 1974 ll7/138.8 F, 76 F, 72, 46 FC; 161/191 Primary Examiner-William D. Martin Assistant Examiner-Janyce A. Bell 5 7 ABSTRACT Organic polymeric shaped objects, such as films, are coated with aluminum orthosphosphate of a specific atom ratio, by a process for their preparation using an aqueous dispersion of phosphoric acid and aluminum chlorhydroxide.

6 Claims, 4 Drawing Figures SEALABLE TOPCOAT' ALUMINUM PHOSPHATE BASE FILM PATENTEDHEEBI'IQM 3.851723 sum 10F 2 I SEALABLE TOPCOAT WM ZLQ1ITw/////A IIl/ MOISTURE VAPOR PERNEABILITY l 2 3 ATOM RATIO (Al/P) PATENn-ju 051:3 1 I974 SHEET 2 OF 2 ATOH RATIO (Al/P) 5 kmmw PROCESS OF COATING POLYMERIC SHAPED OBJECTS WITH ALUMINUM ORTHOSPHOSPHATE CROSS-REFERENCE TO RELATED APPLICATIONS This is a continuation-in-part of copending application Ser. No. 273,301, filed July 19, 1972, and now U.S. Pat. No. 3,821,014, which, in turn, is a continuation-in-part of Ser. No. 182,641, filed Sept. 22, 1971 and now abandoned.

BACKGROUND OF THE INVENTION In the preparation of thermoplastic polymeric films and the like, it is often necessary to modify the permeability or surface characteristics of the finished product to impart characteristics not exhibited by the film itself. Accordingly, many different classes of coating materials have previously been used to modify the characteristics of thermoplastic materials. f the inorganic materials used for coating, continuous, glassy coatings are known to provide excellent moisture and gas barrier properties. However, such materials are generally difficult to apply to the polymer surface, and particularly good barrier materials are continually sought for coatmgs.

SUMMARY OF THE INVENTION The present invention provides a process for preparing coated structures having remarkably improved moisture and gas permeability which are easily and economically prepared.

Specifically, the instant invention provides a process for coating an organic polymeric shaped article with a substantially continuous, gas-impermeable coating on at least one surface thereof by applying to said article an aqueous dispersion of aluminum chlorhydroxide and phosphoric acid wherein the atom ratio of aluminum to phosphorus is less than 1 but at least 0.8, preferably 0.95 to 0.85, and drying the coated shaped article to remove excess water. The dispersion should be coated in an amount sufficient to yield a dried coating weight of about 0.1 to 0.6 gram per square meter.

It is preferred that coated films produced according to the invention further comprise a scalable polymeric coating applied over the phosphate coating.

BRIEF DESCRIPTION OF THE DRAWING FIGS. 1 and 2 are cross-sectional illustrations of representative coated films produced according to the instant invention, the layers of the films not being drawn to scale.

FIG. 3 is a graphical representation of the effect of aluminum orthophosphate compositions on moisture barrier properties.

FIG. 4 is a graphical representation of the effect of aluminum orthophosphate compositions on oxygen barrier properties.

DETAILED DESCRIPTION OF THE INVENTION Shaped articles which can be used in the instant invention include films, containers, bottles, fibers and the like prepared from polyesters, polyalkyls such as polypropylene and polyethylene and copolymers thereof including ionomers, the perfluoro polymer prepared from tetrafluoroethylene and hexafluoropropylene, polyacrylonitrile, polyvinyl chloride, polyvinylidene polyamides, cellulose acetate, cellophane and the like.

Of these, polyester films such as oriented heat-set polyethylene terephthalate and oriented linear polypropylene films are particularly preferred.

The coatings of the present invention are in a substantially glassy, or noncrystalline state. The presence of minor percentages of isolated crystals, however. will not depreciate the moisture and gas barrier properties of the coatings.

Entirely aqueous solutions have been found particularly convenient for most applications, but methanol can be used in a dispersion to replace part of the water. Ethanol can be used in the medium when additives such as melamine formaldehyde resin are not used.

The dispersion is prepared by dissolving the phosphoric acid and aluminum chlorhydroxide in water. By aluminum chlorhydroxide is meant the compound Al (OH) Cl or mixtures of aluminum hydroxide and hydrochloric acid wherein the atom ratio of chlorine to aluminum is between about 0.2 to l and 0.7 to l. The aluminum chlorhydroxide and phosphoric acid should be added in quantities that will result in atom ratios. of aluminum to phosphorus within the required limits, preferably from 0.95 to 0.85.

The concentration of the dispersion is not critical to the invention and can be adjusted to satisfy the requirements of coating apparatus used and final coating thickness desired. In general, the dispersions have a solids content of less than about 6%, when used with kiss or doctor roll coating techniques.

The coating dispersions are applied in such quantities to give a final coating weight of about from 0.1 to 0.6 g./m. to give the desired barrier properties.

Since HCl or chloride ions are formed in'the process,

the removal of the HCl so formed may be necessary to insure the good barrier properties of the invention, and g to achieve wettability of the dispersion onto the polymer surface. Excess I-ICl or chloride ion removal can be effected, if desired, for example, by dialysis or by means of an ion exchange resin such as Rex-yn 201" commercially available from Fisher Scientific Co., but chloride removal is not generally necessary when aluminum chlorhydroxide is used to prepare the coating colloid. In general, it is desirable to remove as much chloride ion as possible without causing the colloidal sol to gel prematurely.

The dispersion can also contain additives such as resins that improve the wettability and adhesion of the coatings to the base to which they are applied. Additives which have been found particularly effective in this application include melamine formaldehyde resins, urea formaldehyde resins, polyethylenimine, and amino acids such as glycine and alanine alone or in combination. Particular resins that can be used include Accobond 3524 trimethylolmelamine modified with iminobispropylamine, Cymel 300 hexamethoxymethylmelamine, and Cymel 450 melamine formaldehyde resin. Of these, Accobond 3524 is particularly preferred because of its wetting characteristics coupled with its protection of coating barrier properties upon heating. In general, up to about 50%, by weight of the phosphate, of such additives can be used in the coating. The additive is conveniently incorporated into the dispersion after the addition of the phosphoric acid and before the addition of the aluminum ions.

The dispersion can be coated onto one or both sides of the substrate using any conventional coating technique including, for example, kiss coating doctor rolls, gravure rolls, immersion coating techniques, spraying, and the like, with or without such expedients as Mayer rods or air doctor knives. The dispersion is preferably applied to the polymeric substrate within a relatively short period after preparation, since extended aging of the dispersion, e.g., longer than 24 hours, permits slight gelation of the coating material. The coating thickness should be such as to give at least 0.02 g./m. of the phosphate on the coated surface, and preferably about from 0.1 to 0.6 g./m. The dispersion can be applied, for better adhesion, to a substrate that has been pretreated.

Treatments which can be used include flame treatment, electrical discharge treatment, as well as acid or alkaline etching of the substrate surface. In general, pretreatment by conventional flame or electrical discharge treatment is preferred. However, when a resin additive is used in the dispersion, the need for treatment of the base surface is frequently eliminated.

After application of the dispersion, the coated substrate is dried at elevated temperatures to remove excess water from the dispersion. Drying times and temperatures will vary widely, depending, for example, on the composition of the base polymeric article, the chemical composition of the coating, the concentration of the coating sol, the coating thickness, and the air flow in the drier. Coated films are conveniently dried by passing through a tower with radiant heat and countercurrent air flow. When organic solvents are used for application of the coatings, substantially all of the sol-' vent should be removed, including the solvated residues that have become chemically attached to the inorganic oxides. The use of radiant heat, however, should be avoided for safety reasons when organic solvents are used. The formation of a good barrier may in some cases be facilitated by the incorporation of small amounts of water, to assist in the removal of bulkier organic residues.

The coatings applied from aqueous systems should be dried to the normal water content ofthe coating, generally less than about 80% by weight of the coating. In general, the amount of water necessary in the coating will decrease with increasing quantities of adhesionpromoting additive, such as a melamine formaldehyde resin, present in the coating. Excessive residual water or incomplete drying can cause discontinuities in the coating. Similarly, drying at an exceptionally fast rate will disrupt the coating structure to give poor barrier properties.

After completion of the drying of the phosphate coating, the coated film can be further treated by priming and applying a sealable topcoat of a polymeric material such as polyethylene, vinylidene chloride polymers and copolymers, and ethylene vinyl acetate. Priming agents that can be used include silanes, polyurethanes, aqueous solutions of melamine formaldehyde resins, and aluminum chlorhydroxide. The top coating can be applied by solvent coating, conventional melt extrusion techniques, hot lamination of a preformed film of the heat-sealable polymer, or polymer dispersions. It has been found that the barrier properties of films of the invention are often markedly improved when sealable polymer topcoats are applied. The improvement so realized is generally substantially greater than the expected additive effect of the barrier properties of the two coatings.

The application of a sealable topcoat to the inorganic coatings of the present invention is particularly preferred for several reasons. In addition to imparting sealability to the surface, the topcoat improves the impact and abrasion resistance of the coated films of the present invention. In addition, the extent to which the coated films can be elongated without depreciating the barrier properties is increased. Most significantly, however, is the further improvement in barrier properties that the sealable topcoat provides. While the mechanism for this improvement is not fully understood, it is postulated that the sealable topcoat seals minute pores or fissures that are present in the inorganic barrier layer. In general, it is recognized that the total permeability (P,) ofa composite varies within the permeability of the individual components (P P etc.) by the following relationship:

It has been found that the improvement realized through the application of a sealable topcoat in accordance with the instant invention decreases gas and water vapor permeability to an extent greater than would be expected from this standard relationship.

After drying of the aluminum phosphate coating, the composition of the coating can be verified using an X-ray emission spectrometer equipped with chromium and tungsten target X-ray tubes, and sample holders permitting irradiation of flat film specimens. In the use of this equipment, any topcoat is first removed from the inorganic coating by immersing the film specimen in tetrahydrofuran or hot toluene, depending on the topcoat involved. The sample is then rinsed until free of polymer and dried in an oven for 10 minutes at I20C. The film specimen isthen irradiated in an X-ray spectrometer equipped with a chromium target tube and pentaerythritol crystal and scan angles. Peaks of intensity of secondary X-rays for aluminum phosphate will be observed at 89.40 for phosphorus and l44.67 for aluminum.

For measurement of the aluminum to phosphorus ratio, standard filmsamples should be prepared having known amounts of aluminum and phosphorus expressed in grams per unit area. The base film from the same lot as that used to make the standards should be available for a blank measurement of secondary radiation. Factors for both aluminum and phosphorus should be separately calculated by measuring the counts per unit time at 89.40 and at l44.67 for the known sample and subtracting the counts at the same setting for the blank and dividing grams per unit area of aluminum by the net counts. The counts per unit time for the test samples at 89.40 and l44.67 should then be measured and the counts from the blank subtracted, after which the result should be multiplied by the factor for aluminum. The procedure is then repeated for phosphorus and the atom ratio of aluminum to phosphorus is obtained by dividing the grams per unit area by the atomic weight and establishing the ratio.

The coatings of the present invention markedly improve the barrier properties of the base films to which they are applied. FIGS. 3 and.4 illustrate these improvements, and represent aluminum orthophosphate coatings applied in thicknesses of about from 0.1-0.4

g./m. to a one-mil, biaxially oriented polyethylene terephthalate film, flame treated to promote adhesion. The coating sampleson which the graphs are based consist essentially of the aluminum orthophosphate, without adhesion promoting additive, and without a sealable topcoat.

As can be seen from these figures, coatings of the present invention provide markedly improved oxygen barriers within the required composition limits, and markedly good water vapor barrier properties within the preferred compositions.

The invention is further illustrated by the following examples. The oxygen permeability characteristics in Examples 1-3 are measured on a cell connected to a mass spectrograph using helium transmission. The helium transmission was calibrated against oxygen transmission in a volumetric cell according to ASTM D- 1434-58. The oxygen permeability in Examples 4-8 are measured using an Ox-tran 100" apparatus commercially available from Modern Controls, Inc. The moisture permeability is measured automatically by a Honeywell Model W825 water vapor transmission tester.

EXAMPLE 1 A colloidal dispersion was prepared from 845 grams of 50% aluminum chlorhydroxide solution, commercially available as Chlorhydrol from Reheis Division, Armour and Co.; 500 grams of 86% phosphoric acid, 28.6 lbs. ofwater, and 190 grams ofAccobond 3524 melamine formaldehyde resin. The dispersion was prepared by first combining the phosphoric acid and one half of the water, adding the melamine formaldehyde resin, and combining this with a mixture of the aluminum chlorhydroxide and the other half of the water. The dispersion, having an aluminum/phosphorus ratio of 0.9, was coated onto both sides of a 0.5-mil biaxially oriented, heat-set polyethylene terephthalate film using doctor rolls.

The coated film was dried in a radiant-heated tower with countercurrent air flow of cfm./ft. of film. The film was dried first in air at about 1 C. for 4 seconds and thenin air at about 170C. for 8 seconds. The final coating thickness was about 0.18 gram per square meter, or about 0.004 mil.

The coated film exhibited a moisture permeability of 20.0 g./l00 m. /hr. and an oxygen permeability of 0.1 cc./l00 in /24 hrs./atm. To establish a control standard, ifa sample of biaxially oriented, heat-set, 0.5-mil polyethylene terephthalate film is coated according to standard commercially used techniques, with a vinylidene chloride copolymer at a thickness of 3.8 g./m. this type of film will typically exhibit a moisture permeability of 50.0 and an oxygen permeability ofl .5. Completely uncoated film of this type typically exhibits a moisture permeability in excess of 225 g./l00 m. /hr. and an oxygen permeability of 10 cc./l00 inf/24 hrs./atm.

EXAMPLE 2 If the procedureof Example 1 is repeated, with the modifications set forth below, the following results will be obtained:

A colloidal dispersion is prepared substantially as in Example 1, except the melamine formaldehyde resin is omitted and the base film is flame treated. The disper sion is applied to give a final coating thickness of 0.2

gram per square meter and is dried for 4 seconds at 115C. and 8 seconds at C.

The coated film is found to have a moisture permeability of 15 g./l00 m. /hr. and an oxygen permeability of 0.07 cc./l00 in. /24 hrs./atm.

1f the film is further coated with vinylidene chloride copolymer containing 92% vinylidene chloride, using conventional priming and coating techniques, so as to give a vinylidene chloride coating of about 3.3 grams per square meter, the moisture permeability is reduced to 11.9 and the oxygen permeability is reduced to 0.024.

EXAMPLE 3-5 If the dispersion of Example 1 is coated onto base films of polypropylene, polyvinylidene chloride-coated cellophane, and polyamide, similar improvements in moisture and/or oxygen barrier properties will be ob tained.

EXAMPLE 6 The general procedure of Example 1 was repeated using methanol as the solvent for the aluminum orthophosphate coating instead of water. The formulation of the aluminum orthophosphate coating bath was 90.7 grams of 50% aluminum chlorhydroxide, 54.0 grams of phosphoric acid, 20.0 grams of 30% melamine formaldehyde resin, and 1,169 milliliters of methanol.

The aluminum orthophosphate coated film was then coated with a topcoating of 240 grams of copolymer of about 93.5% vinylidene chloride, 3% acrylonitrile, 3% methyl methacrylate and 0.5% itaconic acid; 60 grams of Shell Epon resin; 6 grams of paraffin wax; 2 grams of water ground talc; 600 milliliters of toluene; and 1,100 milliliters of tetrahydrofuran. This topcoating was applied in the same manner similar to the aluminum orthophosphate.

The film samples were tested for moisture and gas permeability, and the results obtained were as follows:

AIPO, only AIPO Topcoat Coating weight (g./m. 0.34 4.3 (topcoat) Moisture Permeability (g./1OO mF/hr.) 50 4.2 Oxygen Permeability 0.20 0.02 (co/100 in /24 hr.)

EXAMPLE 7 AlPO. only MPO, Topcoat Coating weight (g./m.') 0.53 3.8 (topcoat) Moisture Permeability 620 13,3 lg./l00 m. /hr'.)

Oxygen Permeability 0.07 0.02

(cc./ 100 in./24 hr.)

Uncoated cellophane generally has a moisture permeability of several thousand and an oxygen permeability, for normal moisture content, of about from 1-5.

EXAMPLE 8 AIPO AlPO Topcoat Coating weight (g./m. 0.33 1.9 (topcoat) Moisture Permeability 18 8.7 (g./l m./hr.) Oxygen Permeability 0.30 0.09

(co/100 in. /24 hr.)

We claim: 1. A process for coating an organic polymeric shaped article with a substantially continuous, gasimpermeable coating on at least one surface thereof by applying to said article an aqueous dispersion of aluminum chlorhydroxide and phosphoric acid wherein the atom ratio of aluminum to phosphorus is less than 1 but at least 0.8, said dispersion being applied in an amount sufficient to yield a dried coating weight of about 0.02 to 0.6 gram per square meter, and drying the coated, shaped article to remove excess water.

2. A process as defined in claim 1 wherein said atom ratio of aluminum to phosphorus is from 0.95 to 0.85 and the coating weight is about 0.1 to 0.6 gram. per square meter.

3. A process as defined in claim 1 wherein said organic polymeric shaped article is a biaxially oriented polyethylene terephthalate film.

4. A process as defined in claim 1 including the step of pretreating the surface of said organic polymeric article to enhance adhesion by flame treatment or electrical discharge treatment.

5. A process as defined in claim 1 wherein excess chloride ion is removed from said dispersion prior to of applying a scalable topcoat over the dried coating. l =l l k 

1. A PROCESS FOR COATING AN ORGANIC POLYMERIC SHAPED ARTICLE WITH A UBSTANTIALLY CONTINUOUS, GAS-IMPERMEABLE COATING ON AT LEAST ONE SURFACE THEREOF BY APPLYING TO SAID ARTICLE AN AQUEOUS DISPERSION OF ALUMINUM CHLORHYDROXIDE AND PHOSPHORIC ACID WHEREIN THE ATOM RATIO OF ALUMINUM TO PHOSPHORUS IS LESS THAN 1 BUT AT LEAST 0.8, SAID DISPERSION BY APPLIED IN AN AMOUNT SUFFICIENT TO YIELD A DIRED COATING WEIGHT OF ABOUT 0.02 TO 0.6 GRAM PER SQUARE METER, AND DRYING THE COATED, SHAPED ARTICLE TO REMOVE EXCESS WATER.
 2. A process as defined in claim 1 wherein said atom ratio of aluminum to phosphorus is from 0.95 to 0.85 and the coating weight is about 0.1 to 0.6 gram per square meter.
 3. A process as defined in claim 1 wherein said organic polymeric shaped article is a biaxially oriented polyethylene terephthalate film.
 4. A process as defined in claim 1 including the step of pretreating the surface of said organic polymeric article to enhance adhesion by flame treatment or electrical discharge treatment.
 5. A process as defined in claim 1 wherein excess chloride ion is removed from said dispersion prior to coating.
 6. A process as defined in claim 1 including the step of applying a sealable topcoat over the dried coating. 